Need help Animating water
If I understand the question, this is fairly easy to do.  In Blender 2.79-2.81 you need three different types of features for water.  These are all selectable in the physics tab under the fluid button.  You need the Domain of the problem which limits where the blender physics engine calculates the water interactions.  This needs to be a rectangular block.  Then you need your water (the ocean in your case) which can be larger than the domain, but only that part in the domain will be animated.  Then you need a fluid obstacle which in your case are the tectonic plates.  When you run the simulation by hitting 'bake' the water will not flow out of the domain unless you put a drain or an outflow in it. You can put an outflow between the plates and set it to drain only when the plates pull apart.

Hope this helps.